In recent years, many attempts have been made to improve fuel-efficiency of vehicles due to emission controls for passenger cars and concerns about the environment etc. One efficient method for improving the fuel-efficiency of a vehicle is to reduce rolling resistance of the tires.
Principal causes of the rolling resistance of a tire include energy loss attributed to inner frictions that occur in various constituents of the tire, such as the tread portion, a sidewall, a carcass, and an inner liner. Accordingly, reducing the energy loss in the tire most effectively helps reduce the rolling resistance of the tire.
The rolling resistance of the tire in particular correlates significantly with loss tangent (tan δ), as measured in the rubber composition constituting the tire tread portion and at 60° C., and lowering the value tan δ results in improved fuel-efficiency of a vehicle.
As one of conventional techniques to lower tan δ at 60° C., it has been suggested to use, in a reinforcing agent to be added to a rubber component, 0 to 100% by mass of silica based on a total amount of the reinforcing agent. However, when silica is added as the reinforcing agent, static charge tends to occur on a rubber surface, which breeds other problems such as occurrence of discharge, interfered radiofrequency reception, and contamination due to accumulation of fine dust or the like.
In an attempt to prevent the static charge build-up on the rubber surface, for example, Patent Document 1 discloses a pneumatic tire coated with anti-static cement. Patent Document 2 also discloses a pneumatic tire in which a highly conductive sheet is interposed in at least one point on a tread circumference.
However, the pneumatic tire of Patent Document 1 needs further review at least in terms of ecological influence. There is also room for improvement in durability of the pneumatic tire of Patent Document 2.